1. Field of the Invention
The embodiment discussed herein are related to a pattern inspection apparatus and a pattern inspection method, and particularly, to a pattern inspection apparatus and a pattern inspection method which are capable of extracting pattern defects difficult to be identified visually in a SEM image.
2. Description of the Related Art
In a lithographic step in a semiconductor manufacturing process, a pattern formed on a photomask is transferred by exposure onto a wafer by using an exposure apparatus. If there is a defect or a strain in the pattern formed on this photomask, exposure accuracy is deteriorated, for example, the pattern is not transferred to a desired position, or the pattern shape becomes inaccurate. In order to prevent such deterioration in exposure accuracy, inspection is carried out on a photomask to find positional errors and defects.
As a method of inspecting a photomask, there is an inspection method utilizing a SEM image of a mask which is taken by a scanning electron microscope. The scanning electron microscope irradiates and scans a region within an electron scanning range with the incident electrons, acquires secondary electrons emitted from a sample by way of a scintillator, acquires SEM image data by converting an amount of the acquired electrons into luminance, and displays a SEM image on a display device.
For example, inspection using a line width of a pattern formed on a mask is carried out in accordance with the following procedures. A predetermined range of a pattern formed on a photomask is displayed on a display unit. Then, an electron beam aimed at a measurement point within the displayed range, and the measurement point is irradiated with the electron beam. Next, a waveform of luminance distribution is acquired based on secondary electrons reflected from the measurement point. Thereafter, pattern edge positions are determined by analyzing the waveform of luminance distribution, and a line width is defined. A judgment is made as to whether or not this line width is within an acceptable error range, thereby determining quality of the photomask.
Meanwhile, defects on the pattern are visually inspected by displaying a pattern image on a monitor. In this visual inspection, it is difficult to carry out the accurate inspection because the determination of the pattern quality varies depending on the inspector.
With respect to this problem, Japanese Laid-open Patent Publication No. 2000-294183 describes the following technique for inspecting shapes of fine patterns. In the technique, reference image data corresponding to variations in the concentration difference and the shape difference is generated based on a plurality of pieces of image data of fine quality product. Then, inter-image calculation is performed on the reference image data and inspection target image data to extract difference image data. Thereafter, the difference image data is compared with a predetermined threshold to extract portions with concentration defect and portions with shape defect.
Further, there is also proposed a defect inspection system which detects pattern defects by using an optical defect inspection device, observes images of the detected defect by using an observation device such as a SEM, and analyzes the defects.
An optical defect inspection device in the defect inspection system described above is able to specify patterns including defects and store positions of the defects as coordinate data. Such a defect inspection system is required to promptly determine the positions of the defects detected by the inspection device within a substrate by using the observation device.
For example, to share a coordinate system between the optical defect inspection device and the observation device such as the SEM, an image of a predetermined pattern on a substrate is detected and the coordinate system is corrected based on a position of this pattern.
However, even if coordinates of a position of a defect pattern are detected by the optical defect inspection device, the following problem may occur. Consider a case where the inspection device and the observation device respectively use different pattern detection methods and have different detection accuracies, for example, a case where the inspection device is of an optical type but the observation device is of an electron beam type as described above. In such a case, the same pattern may not be detected by both the inspection device and the observation device. As a consequence, there may arise a problem in which, even if a coordinate position detected by the optical defect inspection device is used, in the SEM, the coordinate position indicates a position of a pattern without any defects.
Meanwhile, sizes to be defined as defects are becoming smaller in recent years and it is extremely difficult to visually judge such defects in SEM images. In this respect, currently, presence and absence of the defects are judged by using a simulation image to be transferred onto a wafer such as by a wafer plane inspection (WPI) method, and enormous costs are incurred for operations to analyze the defects.
Note that, a defect pattern can be specified even if the coordinates are displaced from each other, provided that the pattern is isolated, for example. However, it is difficult to specify a defect pattern if there are a plurality of similar patterns located adjacent to one another.